This invention relates to polycarbonate resins and, more particularly to an improved polycarbonate resin having prolonged time to failure under conditions of high humidity and mechanical stress.
Polycarbonate resins are polymeric materials which incorporate the carbonate radical ##EQU1## as an integral part of the main polymer chain. In polycarbonate synthesis, a dihydroxy aromatic compound undergoes reaction with a carbonyl compound to yield long chain molecules which consist of alternate aromatic and carbonate groups. An example of such polyarylcarbonate resin is the product of the reaction between phosgene and 2,2-bis(4-hydroxyphenyl) propane (bisphenol A) in the presence of a basic substance such as pyridine. The structure of this polymer is: ##SPC1##
In commercial polycarbonate resins, the number of repeating units n, is such that the molecular weight is from about 25,000 to 75,000.
Polycarbonates are found to be particularly useful in a number of applications because of the relatively high impact and tensile strengths and high softening temperatures exhibited by such resins. For example, U.S. Pat. No. 3,156,107 describes the use of polycarbonate resin to provide a stronger and more heat-resistant exterior coating on the glass envelope of a photoflash lamp; and U.S. Pat. No. 3,770,366 describes a photoflash lamp having a thick, vacuum-formed outer coating of a high-strength thermoplastic, such as polycarbonate resin. Heat is employed in thermoforming process used in applying these coatings to the lamp envelopes. Subsequent cooling of the glass envelope and polycarbonate coating causes the buildup of high tensile forces in the coating because it tends to contract more than the glass. These forces can be reduced somewhat by heating a narrow band of the coating as described in U.S. Pat. No. 3,832,257. It has been found, however, that even such stress relieved coatings can crack and fail in a relatively short time under conditions of high humidity, even when the remaining stresses are within the accepted design limits for the polycarbonate resin used. It should be noted here that unstressed polycarbonate has good resistance toward humidity or even water immersion. In searching for a solution to this aging, or shelf-life, problem under humid conditions, an extensive literature survey failed to shed light on the cause of this unexpected cracking under stress levels allowed by good design practices.
Consideration was then given to the incorporation of a compatible plasticizer into the resin with the anticipation that it might promote relaxation and stretching and thereby relieve the stresses caused by differential contraction between the coating and glass. Evaluation of coatings containing, for example, 20 to 30 parts of a plasticizer to 100 parts of resin did in fact show significantly improved life under humid conditions. The plasticized polycarbonate was quite rigid rather than extensible as had been expected and, therefore, did not function in the manner anticipated. That is, the reduced coating stresses obtained with the plasticized resin were the result of a considerable lowering of the softening temperature needed for thermoforming. Cooling of the coated lamp over a lesser temperature gradient resulted in less stress build up. The shortcoming of this approach, however, was that the introduction of the required amounts of plasticizer resulted in substantial weakening of the coating, when compared to unplasticized polycarbonate. The resulting plasticized polycarbonate did not provide the desired stronger protective coating; more specifically, the plasticized polycarbonate coatings were not consistently better than cellulose acetate lacquer in containment tests with overcharged lamps. In addition, with respect to the preformed polycarbonate sleeves which are vacuum-formed onto the lamp as described in U.S. Pat. No. 3,832,257, the low set point and poor strength at elevated temperatures of the plasticized polycarbonate made extraction from the mold of the injection molded sleeves, a difficult, slow and uneconomical process.